In wavelength division multiplexing systems, optical multiplexers/demultiplexers that multiplex or demultiplex a large number of optical signals of different wavelengths are essential. As the optical multiplexers/demultiplexers, arrayed waveguide grating optical multiplexers/demultiplexers are often used in view of the mass-productivity and stability of those multiplexers/demultiplexers. Conventional arrayed waveguide grating optical multiplexers/demultiplexers will be described below as waveguide type optical devices. The waveguide type optical devices may also be Mach-Zehnder interferometers.
A silicon wafer is used as the waveguide substrate of each arrayed waveguide grating optical multiplexer/demultiplexer, and silica glass is used as the material of the waveguides to be formed on the silicon wafer. FIG. 1 shows a conventional arrayed waveguide grating optical multiplexer/demultiplexer 101. The transmission center wavelength of the arrayed waveguide grating optical multiplexer/demultiplexer 101 is determined by the mathematical equation 1:
                              AWG          ⁢                                          ⁢          transmission          ⁢                                          ⁢          center          ⁢                                          ⁢          wavelength          ⁢                      :                    ⁢                                          ⁢                      λ            0                          =                                            n              c                        ×            Δ            ⁢                                                  ⁢            L                    m                                    [                  Equation          ⁢                                          ⁢          1                ]            
Here, λ0 represents the center wavelength, ΔL represents the difference in length between adjacent channel waveguides in the arrayed waveguide, m represents the diffraction order, and nc represents the refractive index of the channel waveguide.
However, there is a difference in the thermal expansion between the silicon of the waveguide substrate and the silica glass of the waveguide material. Internal residual stress is caused during the process of cooling from a high temperature to room temperature in the manufacture, and waveguide birefringence of approximately 0.0002 is caused due to stress in the arrayed waveguide. This waveguide birefringence causes the transmission center wavelength shift of a TM mode having an electric field perpendicular to the substrate to the longer wavelength, compared with the transmission center wavelength of a TE mode having an electric field parallel to the substrate. In other words, a wavelength shift is caused due to the polarization dependence of the transmission center wavelength. The difference in transmission center wavelength between the TM mode and the TE mode will be hereinafter referred to as the polarization wavelength shift. This polarization wavelength shift is approximately 0.2 nm in an arrayed waveguide grating optical multiplexer/demultiplexer of 0.4 nm in demultiplexing wavelength spacing.
Conventionally, as a polarization dependence eliminating means that eliminates the polarization wavelength shift, there has been a suggested technique in which a polarization mode converter formed with a half-wave plate having a principal axis tilted 45° with respect to the substrate is inserted to an arrayed waveguide, and the TE mode and the TM mode are converted each other (see Patent Document 1, for example). FIG. 2 shows a conventional arrayed waveguide grating optical multiplexer/demultiplexer 102. A polarization dependence eliminating means 17 is placed so that the right and left waveguide structures become symmetric, and the polarization dependence of the difference in optical path length (the product of the effective refractive index and the waveguide length) between right and left is canceled each other for in the manner expressed by the following mathematical equation 2:
                                          AWG            ⁢                                                  ⁢            transmission            ⁢                                                  ⁢            center            ⁢                                                  ⁢            wavelength            ⁢                                                  ⁢                          (              TE              )                        ⁢                          :                        ⁢                                                  ⁢                          λ              TE                                =                                                    1                2                            ×                                                                    n                    CTE                                    ×                  Δ                  ⁢                                                                          ⁢                  L                                m                                      +                                          1                2                            ×                                                                    n                    CTM                                    ×                  Δ                  ⁢                                                                          ⁢                  L                                m                                                    ⁢                                  ⁢                              AWG            ⁢                                                  ⁢            transmission            ⁢                                                  ⁢            center            ⁢                                                  ⁢            wavelength            ⁢                                                  ⁢                          (              TM              )                        ⁢                          :                        ⁢                                                  ⁢                          λ              TM                                =                                                                      1                  2                                ×                                                                            n                      CTM                                        ×                    Δ                    ⁢                                                                                  ⁢                    L                                    m                                                            ︸                                  Left                  ⁢                                                                          ⁢                  side                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  AWG                                                      +                                                            1                  2                                ×                                                                            n                      CTE                                        ×                    Δ                    ⁢                                                                                  ⁢                    L                                    m                                                            ︸                                  Right                  ⁢                                                                          ⁢                  side                  ⁢                                                                          ⁢                  of                  ⁢                                                                          ⁢                  AWG                                                                    ⁢                                  ⁢                                            AWG              ⁢                                                          ⁢              transmission              ⁢                                                          ⁢              center              ⁢                                                          ⁢              wavelength              ⁢                                                          ⁢              difference              ⁢                                                          ⁢                              (                                  TE                  ⁢                                      -                                    ⁢                  TM                                )                            ⁢                              :                            ⁢                                                          ⁢                              λ                TE                                      -                          λ              TM                                =          0                                    [                  Equation          ⁢                                          ⁢          2                ]            
Here, nCTE represents the refractive index of the TE-mode channel waveguide, nCTM represents the refractive index of the TM-mode channel waveguide, λTE represents the transmission center wavelength of the TE mode that the arrayed waveguide grating optical multiplexer/demultiplexer transmits, and λTM represents the transmission center wavelength of the TM mode that the arrayed waveguide grating optical multiplexer/demultiplexer transmits.
On the other hand, as expressed by the mathematical equation 1, λ0 is determined by the optical path length difference. Since the optical path length difference depends on temperature, λ0 also depends on temperature. Therefore, when an arrayed waveguide grating optical multiplexer/demultiplexer is used in an environment where the change in temperature is approximately 10 to 60° C., a control operation is required to maintain the arrayed waveguide grating at a constant temperature. However, if a heater or a Pertier device is used, the arrayed waveguide grating optical multiplexer/demultiplexer becomes large in size, and the cost of the arrayed waveguide grating optical multiplexer/demultiplexer becomes higher. To counter this problem, there has been a suggested method of inserting a temperature-dependence compensator having a thermal index coefficient with a different sign from the waveguides at a rate as to the optical path length difference (see Patent Document 2, for example). FIG. 3 shows a conventional arrayed waveguide grating optical multiplexer/demultiplexer 103. A temperature-dependence compensator having a thermal index coefficient with a different sign from the waveguides is inserted at a rate as to the optical path length difference, and the temperature dependence of the wavelength is canceled each other as expressed by the following mathematical equation 3:
                              AWG          ⁢                                          ⁢          transmission          ⁢                                          ⁢          center          ⁢                                          ⁢          wavelength          ⁢                      :                    ⁢                                          ⁢                      λ            Temp                          =                                                                              n                  c                                ×                                  (                                                            Δ                      ⁢                                                                                          ⁢                      L                                        -                                          Δ                      ⁢                                                                                          ⁢                                              L                        ′                                                                              )                                            m                                      ︸                              AWG                ⁢                                                                  ⁢                waveguide                                              +                                                                      n                  ′                                ×                Δ                ⁢                                                                  ⁢                                  L                  ′                                            m                                      ︸                                                Temperature                  ⁢                                      -                                    ⁢                  dependence                                                  compensator                  ⁢                                                                          ⁢                  inserting                  ⁢                                                                          ⁢                  portion                                                                                        [                  Equation          ⁢                                          ⁢          3                ]            
Here, n′ represents the refractive index of the temperature compensating means, ΔL′ represents the difference in length between adjacent channel waveguides of the temperature-dependence compensator inserting portion, and λTemp represents the transmission center wavelength that the arrayed waveguide grating optical multiplexer/demultiplexer transmits.    Patent Document 1: Japanese Patent Application Laid-Open No. 4-241304    Patent Document 2: International Publication WO98/36299 pamphlet